1. Field of the Invention
This invention relates to a system for determining the mass flow of solids in a flowing fluid-solid mixture, and particularly provides a system for measuring particulate coal or coal derivative mass flow in a flowing transport gas.
2. Description of the Prior Art
In many systems transporting solids or particulate matter in a flowing fluid, it is important to continuously, or intermittently, monitor mass flow in order to properly maintain the process control. This is particularly required in the process streams of coal gasification, liquefaction and fluidized bed combustion systems. Such systems typically feed coal, or coal derivatives such as char, in particulate form, from lock hoppers through starwheel feeders and, in a transport gas, to the process reactor.
While the mass flow rate of the transport gas, prior to mixing with the particulate matter, can be accurately identified with devices such as an orifice, identification of the particulate matter mass flow rate prior to, and subsequent to, mixing with the transport gas, cannot be easily measured with sufficient accuracy. A primary reason for this limitation is that in-line devices, at the operational flow rates, are subjected to a sandblasting type effect which substantially shortens operational life. Additionally, the particulate matter tends to travel at a different flow rate than the transport gas.
Techniques presently used to determine particulate mass flow, upstream of the area of mixing with the transport gas, include lock hopper weight measurement, starwheel feeder speed measurement, and a technique utilizing filtering of lock hopper weight measurement data. Each of these techniques, however, present some undesirable characteristics. Lock hopper weight measurements typically utilize a plurality of strain-gauge type load cells which are summed and monitored. These systems are complicated by mechanical restraints imposed by rigid process system piping and stresses placed upon the hopper by pressurization and thermal expansion, imposing extraneous stresses on the gauges. The strain gauge indications also tend to fluctuate during operation, and systems are utilized to filter or average the indications so as to dampen the fluctuations. Such systems, however, are deficient as a result of an inherent time lag between the raw data and filtered data, inappropriate indications when transfers are made among a plurality of lock hoppers, and the effects associated with lock hoppers generally. Designs can be provided which guarantee a free floating lock hopper and load cell system, and which provide accurate indications, but only with substantial complexity.
With utilization of a starwheel feeder, although the volume of the feeder pockets is known, the pockets are not uniformly filled. Particle size, moisture content and other variables effect the mass transfer rate for a given revolution rate. Additionally, fine material tends to be retained for a perod of time within the feeder pockets and the surrounding casing as a result of pressure differentials, thus providing inaccurate mass flow indications.
Additionally, the existing techniques rely upon upstream measurements, and do not directly indicate any characteristic of the particulate-transport fluid mixture.
It is therefore desirable to provide a substantially on-line arrangement for determining particulate mass flow rate in a transport fluid which alleviates the discussed deficiencies. It is further desirable to provide an arrangement which is simple, reliable and which takes advantage of existing process components.